A scheme has been developed to isolate and characterize all of the enzymes of tetrahydrobiopterin (BH4) biosynthesis from rat brain. The wide distribution of tetrahydrobiopterin and these enzymes in the brain as well as the lack of effect of specific neurotoxins on some of the enzymes in the pathway suggests that tetrahydrobiopterin and/or these biosynthetic enzymes may have other roles than previously thought. Work is now under way to prepare antibodies to each of these enzymes to use for molecular cloning to obtain DNA probes to investigate the physiological roles.